The present invention is directed generally to optical transmission systems. More particularly, the invention relates to controlling optical signal characteristics in optical links including links containing optical amplifiers, such as erbium doped fiber amplifiers (“EDFAs”).
Digital technology has provided electronic access to vast amounts of information. The increased access has driven demand for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems to link the processing equipment.
In response to this demand, communications service providers have turned to optical communication systems, which have the capability to provide substantially larger information bandwidth transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal formats analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.
Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in separate waveguides, i.e. fiber optic strand. The transmission capacity of optical systems was increased by time division multiplexing (TDM) multiple low bit rate, information signals into a higher bit rate signals that can be transported on a single optical wavelength. The low bit rate information carried by the TDM optical signal can then be separated from the higher bit rate signal following transmission through the optical system.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using electromagnetic waves having different wavelengths in the optical spectrum, i.e., far-UV to far-infrared. The pluralities of information carrying wavelengths are combined into a multiple wavelength WDM optical signal that is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems were not initially deployed, in part, because of the high cost of electrical signal regeneration/amplification equipment required to compensate for signal attenuation for each optical wavelength throughout the system. The development of the erbium doped fiber optical amplifier (EDFA) provided a cost effective means to optically regenerate attenuated optical signal wavelengths in the 1550 nm range. In addition, the 1550 nm signal wavelength range coincides with a low loss transmission window in silica based optical fibers, which allowed EDFAs to be spaced further apart than conventional electrical regenerators.
The use of EDFAs essentially eliminated the need for, and the associated costs of, electrical signal regeneration/amplification equipment to compensate for signal attenuation in many systems. The dramatic reduction in the number of electrical regenerators in the systems, made the installation of WDM systems in the remaining electrical regenerators a cost effective means to increase optical network capacity.
EDFAs have proven to be a versatile, dependable, and cost effective optical amplifier in optical transmission system. EDFAs can amplify optical signals over a wavelength range spanning from approximately 1500 nm to 1600 nm. In addition, the amplification is polarization independent and introduces only low levels of channel to channel crosstalk.
However, the characteristics that make EDFAs so useful, also have some negative side effects. For example, because EDFAs provide gain over a wavelength range of the WDM signal, the amplification of the each channel varies with the power of the channel, as well as the total WDM signal power. Therefore, if a channel is added or dropped or a channel has a power variation, all of the channels will experience a gain variation that adversely affects the signal quality.
In addition, EDFAs do not equally amplify each channel within the wavelength range. Thus, when channels are added or dropped or a channel has a power variation, the remaining channels will not only incur gain variations, but the gain variations will generally be nonuniformly distributed across the remaining channels.
The signal degradation resulting from nonuniform gain variations across the wavelength range is compounded in systems having cascaded EDFAs as would be expected. The gain variations, especially in cascaded amplifier chains, can introduce system instability and noise that results in signal distortion, attenuation, and/or loss, and greatly diminish WDM system performance.
Automatic gain control (“AGC”) and automatic power control (“APC”) techniques have been developed to compensate for or suppress channel gain variations in EDFAs. AGC and APC schemes for controlling amplifiers are generally-similar in operation owing to the amplifier relationship that PowerOUT/PowerIN=Gain.
AGC and APC, schemes can generally be categorized as feedback or feed-forward amplifier control schemes depending upon whether the signal is monitored after passing through the amplifier or before entering the amplifier. A general description of AGC and APC schemes can be found in “Erbium-Doped Fiber Amplifiers, Principles and Applications” by Emmanuel Desurvire (1994), pp. 469-480 (“EDFA94”), which is incorporated herein by reference. A brief, more recent summary is provided in “Dynamic Effects in Optically Amplified Networks”, Optical Amplifiers and their Applications (“OAA”) Jul. 21-23, 1997, MC4-1-4, (“OAA97-1”)
Amplifier control in either scheme is generally achieved by one of two methods. The first method is to control the amplifier gain or power by varying the amplifier pump power in response to the monitored signal, such as described in U.S. Pat. Nos. 4,963,832 and 5,117,196. The second method is to introduce a compensating, or control, signal to control the amplifier gain or power, such as described in U.S. Pat. No. 5,088,095 and “Dynamic Gain Compensation in Saturated Erbium-Doped Amplifiers”, IEEE Photonics Technology Letters, v3, n5, pp. 453-455 (1991) (“PT91-1”).
Feedback control can be based on monitoring one or more signal channels or pilot tones, and/or optical noise at the exit of the amplifier, as described in the above-referenced documents. Further examples of pilot tone monitoring can be found in Electronics Letters, Sep. 14, 1989, v25, n19, pp. 1278-1280, (“EL89-1”) and total optical power monitoring can be found more recently in OAA Jul. 11-13, 1996, PDP4-1-5 (“OAA96-1”). In U.S. Pat. No. 5,506,724, ASE associated with a counter-propagating compensating/control channel is monitored to provide feedback control over the control channel.
All optical gain control methods are described in Electronics Letters, Mar. 28, 1991, v27, n7, pp. 560-1, (“EL91-1”) and U.S. Pat. No. 5,239,607. The all optical AGC schemes couple amplified spontaneous emission (“ASE”) from the amplifier through a feedback loop, which is injected into the amplifier input to form a ring laser. The formation of the ring laser locks the gain of the amplifier independent of the input power of the signal at other wavelengths.
Feedback schemes are generally desirable, because the schemes can also account for changes that occur in amplifier performance over time, as well as the input power changes. See “Automatic Gain Control in Cascaded Erbium Doped Fibre Amplifier Systems”, Electronics Letters, Jan. 31, 1991, v27, n3, pp. 193-195, (“EL91-2”).
Conversely, feed-forward schemes do not inherently account for variations in amplifier performance. However, feed-forward schemes in amplifier chains can indirectly account for variations in preceding amplifiers, because the variations will generally evidence themselves in input power variations in successive amplifiers.
An advantage of feed-forward schemes, as discussed in PT91-1, is that the schemes can be implemented without feedback from remote amplifier sites. Therefore, feed-forward control loops can be deployed at logistically convenient locations in a network and operated independently from the amplifiers, as discussed in EL91-2. Also, feed-forward schemes allow the WDM signal to be monitored before or after control channels are combined with the optical signals.
As described in EDFA94 (pages 475-6), it is desirable to control input signal variations at optical switching nodes in optical networks to equalize signals originating from different stations. Either feedback or feed-forward control can be provided to control the signal input power. For example, see Optical Fiber Communication (“OFC”) Conference Technical Digest 1997 TuP4, pp. 84-5 (“OFC97-1”), 22nd European Conference on Optical Communications 1996 (“ECOC96”) 5.49-52 and European Patent Application No. 0829981A2.
While the signal input can be equalized at each node in a network, it generally remains necessary to provide individual amplifier control along an amplifier chain to account for amplifier performance variations. In this regard, EDFA94 (page 472) cautions that “cancellation of transient saturation is achieved by keeping constant not the total EDFA input power, but the sum of all input powers weighted by their respective saturation powers”. However, the author concedes that in WDM systems, the required spectral analysis to control amplifiers based on balancing the amplifier saturation is not practical.
Another shortcoming of current control channel schemes is that the schemes can not be used to protect against large power variations, which may occur in dense WDM systems. Large increases in the control channel power during gain transients can produce spectral hole burning in EDFAs that can degrade the system performance to a greater extent than the gain transients itself. As such, current control channel schemes have limited applicability in WDM systems.
In view of the expanding use of WDM systems and the desire to perform optical networking, it is becoming increasingly necessary to provide more precise and versatile amplifier control. The more highly controllable amplifiers and systems will help drive the further development of high capacity, more versatile, longer distance communication systems.